Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as a general, well controlled, method for heating objects or wafers which are in the form of thin sheets, slabs, or disks. The objects are generally inserted one at a time into a chamber which has at least some portions of the chamber walls transparent to transmit radiation from powerful heating lamps. The transparent portion of the walls is generally quartz, which will transmit radiation up to a wavelength of 3 to 4 microns. These lamps are generally tungsten-halogen lamps, but arc lamps or any other source of visible and/or near infrared radiation may be used. The radiation from the lamps is directed through the transparent portions of the walls on to the surface of the object to be heated. As long as the objects absorb light in the near infrared or visible spectral region transmitted by the transparent portion of the walls, RTP techniques allow fast changes in the temperature and process gas for the different material processes and conditions. RTP allows the "thermal budgets" of the various semiconductor processing to be reduced, as well as allows the production of various metastable states which can be "frozen in" when the material is cooled rapidly.
RTP systems are relatively new. In the last 10 or 15 years, such systems were used only in research and development. The thrust of the work was increasing the temperature uniformity, and developing heating cycles and processes which decreased the thermal budget. Prior art RTP machines can heat unstructured, homogeneous materials in the form of a flat plate or disk, and produce temperature uniformities across the plate adequate for semiconductor processing processes.
The temperature control in current RTP systems is mostly performed by monochromatic (or narrow wavelength band) pyrometry measuring temperature of the relatively unstructured and featureless backside of semiconductor wafers. The results of the temperature measurement are generally used in a feedback control to control the heating lamp power. Backside coated wafers with varying emissivity can not be used in this way, however, and the backside layers are normally etched away or the temperature is measured using contact thermocouples.
A newer method of temperature control is the power controlled open loop heating described in U.S. Pat. No. 5,359,693, which patent is hereby incorporated by reference.
German patent DE42 23 133 C2, hereby incorporated by reference, discloses a method of producing relatively defect free material in RTP machines. Apparatus induced thermal inhomogeneities have been reduced in the last few years because of the demand for more uniform processing. Among the techniques used have been control of the individual lamp power, use of circular lamps, and rotation of the semiconductor wafers with independent power control.
Most RTP machines have a thin rectangular quartz reaction chamber having one end open. The reaction chamber is made from a set of flat quarts plates joined together ("welded") in a manner known in the art. Chambers meant for vacuum use often have a flattened oval cross section. Chambers could even be made in the form of a flat cylindrical pancake. In general, the chambers are used so that the thin objects to be heated are held horizontally, but they could also be held vertical or in any convenient orientation. The reactor chamber is usually thin to bring the lamps close to the object to be heated. The reactor chamber is opened and closed at one end with a pneumatically operated door when the wafer handling system is in operation. The door is usually made of stainless steel, and may have a quartz plate attached to the inside. The process gas is introduced into the chamber on the side opposite the door and exhausted on the door side. The process gas flow is controlled by computer controlled valves connected to various manifolds in a manner well known in the art.
As wafer sizes increase to 300 mm diameter, the quartz "boxes" are increasingly expensive to produce and to use. The amount and size of ancillary equipment inside the RTP chamber is increasing at the same rate. RTP chambers made from different materials, however, have difficulties in that the hot wafers radiate long wavelength infrared radiation, which heats the quartz walls of the RTP systems which are transparent only to visible and near infra-red light. It is thus difficult to seal the transparent walls of the system to the differing material of the non transparent walls, because the differential expansion of the different materials leads to relative motion of the various parts of the RTP chamber. The differential expansion leads to very non uniform forces at different portions of the quartz plates, and such forces can crack the very expensive plates.
Relative motion between the quartz plates and the other parts of the RTP chamber can "scrub" the sealing material and cause contamination of chamber. This contamination is particularly bad for chemical vapor deposition (CVD) of silicon-germanium, where the system must be extraordinarily clean and where the entire system must be "baked" to remove all traces of water vapor from the walls of the system, and where normal sealing materials outgas sufficient material to prevent good CVD growth. Metal gaskets must be used and must be compressed with high pressures, and upon non uniform heating of the quartz plates, the plates may crack.